We have recently discovered a sexually dimorphic motor nucleus, the spinal nucleus of the bulbocavernosus (SNB), which innervates penile striated muscles in male rats. The SNB and its target muscles are nearly absent in females. The proposed series of experiments will study the sexual differentiation of this behaviorally relevant motor nucleus and its target muscles. Because of the relative simplicity of neuromuscular systems, and because the SNB is likely to be intimately involved in sexually dimorphic copulatory behavior, this is an attractive model system for study of the ontogeny sex differences in central nervous structures involved in behavior. By injecting various steroid hormones into pre and post-natal rats of various ages, we will ascertain which steroids are effective in determining the presence or absence of the SNB and its target muscles, and when they exert such an action. Using steroid autoradiography, we will investigate the development of steroid accumulation in the SNB and its target muscles, and relate this to the effects of steroids on development. Using thymidine autoradiography and other histological techniques, we will study the timing of neuronal birth and development of the SNB in normal males and females, and relate this to the timing of steroid effects in this system. Using implants of steroids, we will attempt to discover the sites of steroid action on the development of this system, which will also have important implications concerning the mechanisms of steroid action.